Sprockets are coupled with a rear wheel of a bicycle through a hub. This comprises a first body, rigidly fixed to the rim of the bicycle through spokes, and a second body, rigidly coupled with the sprockets and able to rotate freely with respect to the first body in a direction of rotation, and to make it rotate in the opposite direction, thus giving the rear driving wheel its forward motion. In the technical jargon, this second body is called “free body.”
Since a bicycle is a means of transport using muscular propulsion, there is a general requirement for the power transmission system from the cyclist to the driving wheel to allow the least tiring possible.
As known, the combination of a guide toothed wheel with a small diameter with a sprocket with a large diameter allows demanding climbs to be ably handled. However, this same combination, over flat land or going downhill, is disadvantageous since the cyclists energy is dissipated, due to the fact that the cyclist is obliged to pedal quickly while the bicycle moves forwards slowly.
In order to make the aforementioned combination more suitable for the route to be made, it is known to equip the bicycle with a plurality of guide toothed wheels and with a plurality of sprockets, which can be combined with each other based upon requirements, through appropriate gearshifting devices.
Over the past few years the number of transmission ratios available in gearshifting devices has progressively increased and on the market currently there are groups of wheels with ten sprockets and groups of wheels with three guide wheels.
Above all in the field of racing bicycles, the progressive increase in the number of toothed wheels requires an ever-increasing search to reduce the weight of the assembly of wheels.
For this reason, assemblies of sprockets have been made comprising a plurality of sprocket support members that carry respective toothed wheels, in the form of circular toothed crowns: since the support members are made from a lighter material than the material of the toothed wheel, the desired reduction in weight of the assembly is obtained.
Since bicycle components have reached a high degree of standardization, the characteristic size of components like the frame, the gearshifting device and the free body are now almost fixed. In the prior art, some limits have been established in the maximum number of guide toothed wheels and of sprockets that can be mounted on standard sized bicycles. These limits currently seem insurmountable without modifying the standardized size of the bicycle components, in particular without increasing the axial size of the aforementioned groups of guide toothed wheels and of sprockets.
Indeed, keeping fixed the aforementioned standardized axial size, an increase in the number of toothed wheels would result in the need to arrange such toothed wheels in positions ever closer one to the other and the space available for the support members of the toothed wheels would become increasingly small, which may thus no longer have the characteristics of rigidity and strength necessary to support the toothed wheels.